Delta shower valve leak

I have a Delta shower head that continues to leak. The whole shower was installed 3 +- 1yr (new house construction). I pulled the shower head off, left it off for a while, and realized that it continued to drip. I therefore assume the problem lies in the valve (or a gasket). I turned off the water and tried to pull the valve apart, but didn't want to destroy anything so I stopped when it required a bit of force. I've got some pics, but my question is what part do I need to fix the leak? And ultimately, how do I fix it?

P.S. I've never used this site before...Let me know if my link doesn't work for my pictures. I can find another way to post them.

Thank you everyone! This is a load off. Quick follow up questions... Do I need any tools for the cartridge removal or does it just pull out (after the brass ring is unscrewed)? And would Home Depot or Menard's have one of these kits?

The new cartridge ( Delta RP19804) will have a slightly different design on the hot limit stop. It IS the right part for your shower. Delta just redesigned it, but it does work on all 1300 models before 2005.

Thank you again for all your help. I went to Home Depot and got the spring/gasket repair kit. It was only $4 vs $40 for the cardridge. I replaced it and put it all back together... and no more leak! Thank you again!!!

I have the same faucet a Delta Monitor about 12 yrs old. I recently replaced the cartridge assembly as it was leaking. It's now leaking again. Problem I had before was removing the handle. I had to cut it. Now I'm able to remove the set screw which I was not able to do last time (6 months ago). But... I can't get the handle off. Since it's only been on for 6 months it should not be corroded. Any suggestions on how to get the handle off. Is there a puller tool?